packet: in packet_snd start writing at link layer allocation

Packet sockets allow construction of packets shorter than
dev->hard_header_len to accommodate protocols with variable length
link layer headers. These packets are padded to dev->hard_header_len,
because some device drivers interpret that as a minimum packet size.

packet_snd reserves dev->hard_header_len bytes on allocation.
SOCK_DGRAM sockets call skb_push in dev_hard_header() to ensure that
link layer headers are stored in the reserved range. SOCK_RAW sockets
do the same in tpacket_snd, but not in packet_snd.

Syzbot was able to send a zero byte packet to a device with massive
116B link layer header, causing padding to cross over into skb_shinfo.
Fix this by writing from the start of the llheader reserved range also
in the case of packet_snd/SOCK_RAW.

Update skb_set_network_header to the new offset. This also corrects
it for SOCK_DGRAM, where it incorrectly double counted reserve due to
the skb_push in dev_hard_header.

Fixes: 9ed988cd5915 ("packet: validate variable length ll headers")
Reported-by: syzbot+71d74a5406d02057d559@syzkaller.appspotmail.com
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -2903,13 +2903,15 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
 	if (skb == NULL)
 		goto out_unlock;
 
-	skb_set_network_header(skb, reserve);
+	skb_reset_network_header(skb);
 
 	err = -EINVAL;
 	if (sock->type == SOCK_DGRAM) {
 		offset = dev_hard_header(skb, dev, ntohs(proto), addr, NULL, len);
 		if (unlikely(offset < 0))
 			goto out_free;
+	} else if (reserve) {
+		skb_push(skb, reserve);
 	}
 
 	/* Returns -EFAULT on error */